Contrast in optical microscopy

The situation is somewhat comparable to positive and negative phase contrast in optical microscopy. The lens imperfections have been used to Introduce a phase shift of jt/2 in the same way as the quarter wavelength ring in the optical microscope. Only beams passing through the "window in which the condition sin X =- I is met, interfere with the required phase relationship that causes the image to represent maxima in projected potential as dark areas. 

The resolution in TEM is limited by lens aberrations. In contrast to optical microscopy, where by serially ordering concave and convex lenses, aberrations can be compensated and hence the wavelength of the radiation is resolution limiting in TEM lens aberrations cannot be compensated since concave electron lenses are not feasible. The objective lens is the crucial part for image defining the microscope s resolution. The quality of the objective lens is described by the constants of spherical Cs(" 0.5-3mm) and chromatic aberration Cc ( 1-2 mm). Recently, microscopes equipped with complex correctors for the spherical aberration have become available. ... 

In optical microscopy the contrast between different components arises from differences in their refractive indices. Ice cream is too opaque for its structure to be observed. However, samples can be prepared so that the ice crystals, air bubbles and fat droplets can be visualized separately. Lactose crystals, if any are present, can be clearly observed using crossed polars. They have a characteristic arrowhead shape that makes them easily distinguishable. 

Scanning electron microscope (SEM) systems are ideal for a pictorial representation of morphological features of single layers of multilayered paint samples (particularly those not well contrasted by optical microscopy as the white ones) obtained by the contrast of secondary (SE) and backscattered electrons (BSE). An important advantage of SEM is the ability to analyze even the smallest smears for this purpose the smears are lifted from the underlying material using double-sided adhesive tape transferred to the sample holder (e.g., slide). Twenty micrometers microtome sections of paint cross-sections are recommended. In order to avoid electrical charging the samples have to be carbon coated or transferred into the chamber of a low vacuum system. 

The above definition of a homogeneous microstructure does not pertain to pores and structural components which may be visible by contrast differences in optical microscopy with polarized light. As a consequence, glass-like carbon with visible pores is still an agranular carbon. The same is true, for instance, for pyrolytic carbon with preferred orientation, such as conical or lamellar structures, visible in optical microscopy with polarized light. The use of the term agranular carbon is not restricted to bulk materials of a minimum size. Only particulate carbon should be excluded even if the isolated particles exhibit a homogeneous microstructure. 

In the second half of the 20th century, a number of advanced variants of optical microscopy were invented. They include phase-contrast microscopy (invented in France) and multiple-beam interference microscopy (invented in England), methods... 

As indicated above in chiral mesophases, the introduction of a functional group in mesogenic stmctures offers the opportunity to achieve functional LCs. With this aim, mesomorphic crown-ether-isocyanide-gold(I) complexes (26) have been prepared recently [38]. The derivatives with one alkoxy chain show monotropic SmC mesophases at or close to room temperature. In contrast, the complexes with three alkoxy chains behave as monotropic (n = 4) or enantiotropic (n > 4) LCs. The structure of the mesophases could not be fully eluddated because X-ray diffraction studies in the mesophase were unsuccessful and mesophase characterization was made only on the basis of polarized optical microscopy. These complexes are luminescent not only in the solid state and in solution, but also in the mesophase and in the isotropic liquid state at moderate temperatures. The emission spectra of 26a with n=12 were... 

The lateral structure was studies by optical microscopy, and contrast arises from the fact that after irradiation of the samples with the ion beam in the TOF-ERDA apparatus, the PBrxS-rich structures appear darker than those rich in dPS, giving an excellent contrast (Figure 4.30). 

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